THIS invention relates to a thermal spraying method for producing a hard coating on a substrate, and to thermal spraying apparatus which can be used for producing metallic or cermet coatings on a substrate.
Arc metal spraying is used in industry to produce coatings on substrates by generating an arc between feedstock electrodes. The molten feedstock is divided into small particles of molten material by an atomising gas jet. These molten particles are propelled by the gas jet onto the substrate to be coated. The fineness of the particles is determined, inter alia, by the velocity of the atomising gas jet.
It is an object of the invention to provide a thermal spraying method which can be used to produce hard coatings with desirable properties, and an alternative thermal spraying apparatus.
According to a first aspect of the invention a method of forming a coating on a substrate comprises the steps of:
providing a feedstock material containing titanium;
atomising the feedstock material in the presence of nitrogen; and
spraying the atomised material onto a substrate to form a coating comprising titanium nitride on the substrate.
The coating may additionally comprise oxides and carbides of titanium.
The feedstock material is preferably atomised by generating an arc between at least two feedstock elements.
Preferably, at least one of the feedstock elements is a titanium wire which is fed towards a point of intersection between the feedstock elements where the arc is generated.
The point of intersection is preferably located within a throat of a nozzle, the method including supplying a nitrogen rich gas under pressure to the throat of the nozzle to assist in expulsion of atomised particles therefrom.
The gas is preferably supplied to the throat of the nozzle at a pressure sufficient to generate choked gas flow in the throat.
The gas will typically be air.
At least one of the feedstock elements may be a wire comprising a metal selected to have suitable properties as a binder of the titanium nitride in the coating, such as nickel.
According to a second aspect of the invention there is provided thermal spraying apparatus comprising:
a nozzle defining a throat having an inlet and an outlet;
at least first and second guides arranged to guide respective feedstock wires via the inlet towards a point of intersection in the throat, so that connection of the wires to a power supply causes an arc in the throat between the wires, creating molten particles which are expelled from the outlet.
The throat may comprise a tubular bore which substantially surrounds the point of intersection of the two feedstock wires.
The diameter of the throat is preferably substantially constant along its length.
The length of the throat is preferably approximately equal to its diameter.
Preferably, the point of intersection is between a point located about midway along the length of the throat and the outer end of the throat.
The nozzle preferably defines a gas flow path which is aligned with the axis of the throat, so that gas under pressure can be supplied to the inlet between the feedstock wires to assist in expulsion of molten particles from the outlet.
The nozzle may define a chamber inwardly of the throat, the chamber having an inner wall which has an average internal diameter several times greater than that of the throat and which tapers inwardly towards an inner end of the throat.
The inner wall of the chamber preferably joins the inner end of the throat at an angle of approximately 45xc2x0.